This invention relates generally to improvements in prosthetic joints such as hip prostheses and the like. More specifically, this invention relates to an improved prosthetic joint wherein articulating surfaces are encapsulated within a semipermeable membrane which prevents undesired migratory distribution of prosthesis-related particulate while permitting natural fluid circulation for prosthesis lubrication.
Artificial or prosthetic devices for implantation into animals, particularly humans, have been the subject of extensive research and development efforts for many years. Such prosthetic devices have typically comprised one or more implant components formed from a relatively biostable material or materials having selected structural properties and unique shapes to replace all or part of selected bone structures, such as an anatomical joint including, for example, hip joints, knee joints, shoulder joints, etc. The implant components are installed by surgical access to the subject bone or joint region, and by resection of one or more bone surfaces to accommodate direct implant component attachment to the bone.
More specifically, in the example of a hip prosthesis, the hip joint of a patient is accessed surgically to permit removal of the head and neck of the patient's femur to expose the internal medullary canal. A prosthetic component having an artificial femoral head and neck is then implanted by seating an elongated stem of the prosthesis component into the medullary canal. In many procedures, a second prosthetic component is implanted into the patient's acetabulum and includes a typically plastic bearing cup to engage the head of the femoral component and thereby form a reconstructed artificial hip joint. In the past, the femoral and/or acetabular prosthetic components have been secured to adjacent patient bone by use of a bone cement, such as methylmethacrylate. In other prosthesis design, porous bone ingrowth coatings have been provided on the prosthetic components for noncemented fixation to patient bone by ingrowth and/or resorption of patient bone and/or tissue.
During normal post-surgical patient ambulation, it is known that a substantial quantity of microscopic and macroscopic particulate debris is generated by the prosthetic joint. More specifically, such particulate debris is produced at articulating surfaces of the joint. By way of example, in a hip prosthesis, the articulating surfaces defined by the femoral head formed typically of a cobalt-chrome or titanium alloy which mates with the plastic acetabular bearing cup produces a substantial quantity of plastic-based particles in the course of normal mechanical wear. These particles can be distributed systemically, and the long term toxicity and oncogenic properties of such particles are unknown.
In addition, localized distribution of particulate contributes to osteolysis or direct bone resorption around the prosthesis, leading to loosening, pain and eventual failure of the prosthesis. Revision surgery for a failed prosthesis is more difficult, more costly, and potentially involves greater risk of complications.
In addition, particulate debris can be generated at the prosthesis-bone interface. This type of particulate can include bone cement fragments and/or small ceramic-based or metal-based debris from porous bone ingrowth materials. Prosthesis-related particles of these types are typically relatively large in size and significantly harder than the plastic bearing materials used at articulating surfaces of the prosthetic joint. Accordingly, circulation of particulate debris from the prosthesis-bone interface to the articulating surfaces of the joint can contribute to significant increases in wear rate.
The above-discussed problems of prosthesis-related particulate debris have resulted in attempts to encapsulate the articulating surfaces of the prosthetic joint. In one approach, as disclosed in U.S. Pat. No. 3,683,421, the articulating joint surfaces are contained within a bellows-like seal to define a closed pocket for containing a synthetic lubricant used to lubricate the articulating surfaces. Unfortunately, however, a satisfactory lubricant for this purpose has not been available or otherwise approved for human implantation. In an alternative design approach as described in U.S. Pat. No. 3,739,403, the articulating joint surfaces are contained within a ball-shaped shell having small ports to permit circulation of natural body fluids past the articulating surfaces for lubrication. The encapsulating shell is intended to prevent ingrowth of large body tissues into the region of the articulating surfaces. Particulate circulation to or from the articulating surfaces, however, is not prevented.
In other joint capsule designs, a semipermeable membrane has been proposed for use in capturing prosthesis-related particulate debris, yet still allow for natural circulation of body fluids to the joint. See, for example, U.S. Pat. Nos. 4,731,088 and 4,822,368, as well as European Patent 0,346,294. These designs, however, use bent wires or drawstrings to hold a fabric-based membrane in place. Such membrane attachment structures must be subjected to significant tension in order to provide an effective particulate seal, but high tension forces increase the risk of membrane rupture at the point of attachment and also increase the risk of failure of the attachment device. If the attachment device comes loose, it can become trapped at the articulatory surfaces to cause dislocation in some cases. Moreover, in these devices, the fabric-based membrane is loosely mounted about the prosthetic joint so that the membrane can also be pinched between articulatory surfaces to result in membrane rupture and possible joint dislocation.
There exists, therefore, a need for further improvements in prosthetic joints of the type intended for human implantation, wherein undesired circulation of prosthesis-related particulate to or from articulating surfaces of the joint is substantially prevented by a semipermeable membrane which permits circulation of natural body fluids to those articulating surfaces for purposes of lubrication, while safeguarding the membrane against significant risk of failure and/or entrapment between articulating surfaces. The present invention fulfills these needs and provides further related advantages.